内存管理API之get_user_pages

long get_user_pages(unsigned long start, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages,
struct vm_area_struct **vmas)
用于将用户空间的页映射到内存，并返回它们页结构的指针，即get_user_pages的第四个形参struct page **pages
其源码分析如下：
long get_user_pages(unsigned long start, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages,
struct vm_area_struct **vmas)
{
#可见这个函数在带当前进程中映射内存
return __get_user_pages_locked(current, current->mm, start, nr_pages,
pages, vmas, NULL, false,
gup_flags | FOLL_TOUCH);
}
static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
struct mm_struct *mm,
unsigned long start,
unsigned long nr_pages,
struct page **pages,
struct vm_area_struct **vmas,
int *locked, bool notify_drop,
unsigned int flags)
{
long ret, pages_done;
bool lock_dropped;
#本例中locked 为null
if (locked) {
/* if VM_FAULT_RETRY can be returned, vmas become invalid */
BUG_ON(vmas);
/* check caller initialized locked */
BUG_ON(*locked != 1);
}

if (pages)
flags |= FOLL_GET;

pages_done = 0;
lock_dropped = false;
for (;;) {
#从start 开始映射nr_pages的页，将页指针保存在pages中，这里的ret表示映射成功页的个数
ret = __get_user_pages(tsk, mm, start, nr_pages, flags, pages,
vmas, locked);
if (!locked)
/* VM_FAULT_RETRY couldn't trigger, bypass */
return ret;

/* VM_FAULT_RETRY cannot return errors */
if (!*locked) {
BUG_ON(ret < 0);
BUG_ON(ret >= nr_pages);
}

if (!pages)
/* If it's a prefault don't insist harder */
return ret;
#这个if 条件成立，说明__get_user_pages 成功映射了ret个页
if (ret > 0) {
nr_pages -= ret;
pages_done += ret;
if (!nr_pages)
break;
}
if (*locked) {
/* VM_FAULT_RETRY didn't trigger */
if (!pages_done)
pages_done = ret;
break;
}
/* VM_FAULT_RETRY triggered, so seek to the faulting offset */
pages += ret;
start += ret << PAGE_SHIFT;

/*
* Repeat on the address that fired VM_FAULT_RETRY
* without FAULT_FLAG_ALLOW_RETRY but with
* FAULT_FLAG_TRIED.
*/
*locked = 1;
lock_dropped = true;
down_read(&mm->mmap_sem);
#再从start 开始映射一个页，正常情况下这里会failed。
ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
pages, NULL, NULL);
if (ret != 1) {
BUG_ON(ret > 1);
if (!pages_done)
pages_done = ret;
break;
#正常情况下for 死循环会从这里退出
}
#如果程序跑到这里，说明第一次映射nr_pages 页的时候failed了。但是第二次调用__get_user_pages
#映射单页确成功了。因此总的要映射的page 数减一，且start起始地址加一个page 后，重新开始映射.
nr_pages--;
pages_done++;
if (!nr_pages)
break;
pages++;
start += PAGE_SIZE;
}
if (notify_drop && lock_dropped && *locked) {
/*
* We must let the caller know we temporarily dropped the lock
* and so the critical section protected by it was lost.
*/
up_read(&mm->mmap_sem);
*locked = 0;
}
#这里的pages_done 表示映射成功的页的个数.
return pages_done;
}